Hardcopy devices, such as photocopiers, scanners and printers, are capable of reproducing color images from color documents and photographs. To reproduce the color images, the images from the color documents or photographs are sensed and reproduced based upon tristimulus values, whose amplitude is proportional to radiance, but whose spectral composition is carefully chosen according to the principles of color science. Tristimulus values in typical imaging systems are represented as red, green and blue (RGB), which are linear light values. In the typical imaging systems, the RGB tristimulus values are subjected to a nonlinear transfer function, such as gamma correction, that mimics the lightness response of vision. The resulting non-linear light values may be represented as R′G′B′ tristimulus values.
A digitized color image is represented as an array of pixels, where each pixel contains numerical components that define a color. The systems that may be used for image coding include the linear RGB and nonlinear R′G′B′ systems described above. Other image coding systems include nonlinear cyan, magenta and yellow (CMY), nonlinear CMY and black (CMYK), and derivatives of these coding systems. Cyan in tandem with magenta produces blue, cyan with yellow produces green and yellow with magenta produces red. The CMY values are typically derived from the RGB or R′G′B′ values, and the K value is typically derived from the CMY values.
In a color image, there are regions that include black and regions that do not include black. These regions can be referred to as black color regions and non-black color regions. The non-black color regions may correspond to regions of light tones, such as regions incorporating skin color. After an image is scanned, RGB data is generated and can be converted to CMY data. The CMY data is then used to determine the K data or black level for each pixel. In a conventional system, the K data or black level is generated regardless of whether the pixel is in a black color region or a non-black color region. As a result, when the image is reproduced from the CMYK data, non-black color regions may be printed with some black. For example, the reproduction of a skin color region may include black dots, which are not close to the real skin color.